1. Field of the Invention
The present invention relates to a variable magnification optical unit useful for the slitwise exposure type copying machine and other similar image forming apparatus.
2. Description of the Prior Art
In an optical apparatus in which the object plane and the image plane are fixed and a fixed focus lens is used as the lens system to form an image of the object plane on the image plane there is a problem in changing the magnification of the focused image. In such optical apparatus, the magnification change is generally achieved by shifting the focusing (image-forming) lens system along its optical axis. However, when the focusing lens system is moved, it is required to maintain the optically conjugated relation between the object plane and the image plane with respect to the focusing lens system. To provide the variable magnification facility, therefore, the problem is in how to maintain the conjugated relation between the object plane and the image plane while shifting the focusing lens system.
A solution to the problem according to the prior art is disclosed in Japanese Utility Model Application Publication No. 12,464/1965. According to the known solution, the optically conjugated relation between the object plane and the image plane is maintained by shifting mirrors provided on the object space side when the focusing lens system is moved for magnification change. Concrete arrangements of such variable magnification optical apparatus according to the prior art are shown in FIGS. 1 and 2 for the purpose of illustration.
In the arrangement shown in FIG. 1, there are provided mirrors 1 and 2 for correcting the optical path length. When the magnification is changed, the mirrors 1 and 2 are shifted in the directions of arrows A.sub.1 and B.sub.1 respectively by the same distance without changing the principal optical path, and the lens 3 is shifted to the position 3' along the optical axis. Mirrors 4 and 5 are used to deflect the optical path and to obtain the space for the photosensitive medium 7. The use of deflecting mirrors 4 and 5 serves to reduce the horizontal size of the whole apparatus. The image of an original 6 is projected slitwise on the photosensitive drum 7 through the optical system.
In the arrangement shown in FIG. 2 there are provided mirrors 8 and 9 for correcting the optical path length. When the magnification is changed, the mirrors 8 and 9 are shifted in the direction of arrow A.sub.2 and the lens 3 is shifted to the position 3' along the optical axis. Mirrors 10 and 11 are stationary mirrors for deflecting the optical path.
Another solution to the above problem is disclosed in Japanese Patent Application Laid-Open No. 9,436/1977. According to the solution, mirrors are provided on the image space side of the focusing lens system. With the movement of the focusing lens system for changing the magnification, these mirrors are also moved to change the optical path length between the object plane and the image plane thereby maintaining the optically conjugated relation between the two planes with respect to the focusing lens system. More concretely, two mirrors are provided on the image space side of the focusing lens system. When the magnification is changed, one of the two mirrors is rotated by a certain determined angle of .theta.. The other mirror is not only rotated by the same angle .theta. but also shifted translationally in a determined direction. Under the cooperative action of these two mirrors, the optical path length between the object plane and the image plane is changed while maintaining constant the incident optical path and the exit optical path of the beam to and from the two mirrors.
However, the above described known optical apparatus provided with the variable magnification facility have some drawbacks. In the prior art apparatus, the correction of the optical path length between the object plane and the image plane as required when the single focus image-forming optical system is moved along the optical axis, is achieved by a group of mirrors which are particularly provided on the object space side or on the image space side of the image-forming optical system. This means that within the apparatus there are always present other mirrors which are never contributable to the necessary correction of the optical path length. For the transfer type of copying machine it is required to form a mirror image of the original in the object plane on the photosensitive medium which is the image plane. If mirrors for deflecting the optical path are used in the apparatus to realize a compact construction of the apparatus, there are needed not an odd number but an even number of mirrors in the optical path as a whole. Therefore, in the variable magnification optical apparatus according to the prior art there were required at least four mirrors in total. To realize a compact construction of the apparatus a further number of mirrors were generally needed in addition to the four mirrors.
Although the compactness of the apparatus can be further improved with increasing the number of mirrors for deflecting the optical path, this brings about another problems of cost increase and more difficult optical adjustment. In this point of view it is desirable that the number of mirrors which are not contributable to the correction of the optical path length should be as small as possible.